FIGS. 1A and 1B are a plan view and a profile view (partially respectively of a cross-section) showing an electrode for a conventional vacuum breaker as disclosed in, for example, Japanese Patent Application Laid-Open No. 30174/80.
This electrode includes a generally disk-shaped member having a central flat part 1 with contact function and peripheral tapered parts 2 shaped like the vanes of a windmill for acting as a current-breaking function.
From the flat part 1 to the outer rim of the tapered parts 2, there are several spiral slots 3 extending outwards and inclined at an angle to the radial direction of the electrode.
The electrode further includes an electrode rod 5 connected to the center of the rear surface (lower surface as seen in FIG. 1B) of the disk-shaped member 10.
In the vacuum breaker having the electrodes described above, when a pair of electrodes which have the flat parts 1 in contact, are separated, an arc is set up between the flat parts 1. This arc is driven along to the current path formed by the electrode, and driven outwards along the direction of the electrode. The arc so driven reaches the spiral slot 3, and moves along the spiral slot 3. At this point, the arc is subject to a composite force composed of the circumferential direction force and the radial direction force, and the electrode surface is thereby rotated. When this occurs, the arc rotates over the whole surface of the electrode, and local heating of the electrode does not result.
By increasing the length of the electrode in the circumferential direction, or the diameter of the electrode, the area over which the current flows is increased so that the current-breaking capacity of the vacuum breaker will be increased. The width or shape of the spiral slot 3 may also affect the current-breaking capacity. In the reference mentioned above, it is noted that for vacuum breakers having a current rating of 8 KA or more, the width of the spiral slot should be at least 1.5 mm.
In conventional vacuum breakers of the above described type, however, it was found that the breaking capacity did not increase linearly with the diameter of the electrode. This was a major obstacle which prevented vacuum breakers from becoming more compact.